Ferritic stainless steel

ABSTRACT

The ferritic stainless steel has a chemical composition containing, by mass %, C: 0.003% or more and 0.012% or less, Si: 0.30% or more and 0.60% or less, Mn: 0.10% or more and 0.35% or less, P: 0.040% or less, S: 0.020% or less, Cr: 17.0% or more and 19.0% or less, Ni: more than 0.10% and 0.30% or less, Ti: 0.10% or more and 0.40% or less, Nb: 0.005% or more and less than 0.050%, Mo: 0.20% or less, N: 0.005% or more and 0.015% or less, Cu: 0.3% or more and 0.5% or less, Mg: less than 0.0005%, and the balance being Fe and inevitable impurities.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2012/007593, filedNov. 27, 2012, which claims priority to Japanese Patent Application No.2011-261799, filed Nov. 30, 2011, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

FIELD OF THE INVENTION

The present invention relates to ferritic stainless steel with corrosionresistance, in particular, excellent corrosion resistance in a weld zonewhich is formed when the ferritic stainless steel is welded togetherwith austenitic stainless steel, and with excellent surface quality.

BACKGROUND OF THE INVENTION

Among stainless steels, SUS304 (18% Cr-8% Ni) (Japanese IndustrialStandard, JIS G 4305) which is a kind of austenitic stainless steel iswidely used because of its good corrosion resistance. However, this kindof steel is expensive because a large amount of Ni is contained therein.Therefore, the stainless steel according to Patent Literature 1 wasdeveloped as a steel grade having a good corrosion resistance equivalentto that of SUS304.

Patent Literature 1 discloses a ferritic stainless steel having achemical composition containing, by mass %, C: 0.03% or less, Si: 1.0%or less, Mn: 0.5% or less, P: 0.04% or less, S: 0.02% or less, Al: 0.1%or less, Cr: 20.5% or more and 22.5% or less, Cu: 0.3% or more and 0.8%or less, Ni: 1.0% or less, Ti: 4×(C % +N %) or more and 0.35% or less,Nb: 0.01% or less, N: 0.03% or less, C+N: 0.05% or less, and the balancebeing Fe and inevitable impurities.

In addition, since ferritic stainless steels of JIS- SUS444,JIS-SUS430J1L, and the like have characteristics such as having lowerstress corrosion cracking sensitivity than austenitic stainless steelsand not containing Ni whose price varies largely, these stainless steelsare widely used as materials for exhaust system parts of automobiles andwater tanks and building materials.

However, since ferritic stainless steels generally have lessformability, in particular ductility, than austenitic stainless steels,austenitic stainless steels are used as materials for parts which aretoo difficult to form from ferritic stainless steels. Therefore, thereare many cases where one component is formed by connecting parts made ofaustenitic stainless steels and that of ferritic stainless steels. Inthese cases, parts are joined together by performing welding in mostcases, and, among welding methods, TIG welding (Tungsten Inert Gaswelding) is mainly used, and good corrosion resistance is required forthe weld zone as well as the base metal.

Patent Literature

[PTL 1] Japanese Patent No. 4396676

[PTL 2] Japanese Patent No. 2842787

SUMMARY OF THE INVENTION

The ferritic stainless steel according to Patent Literature 1 has goodcorrosion resistance in a weld zone in the case where steels of the samegrade are welded. However, there is a problem in that the corrosionresistance in a weld zone is less than that of base metals in the casewhere the ferritic stainless steel is welded together with another kindof steel such as SUS304 by performing TIG welding. This is because,since C or N in the steels combines with Cr in heat cycles at thewelding so that Cr-carbides such as Cr₂₃C₆ or Cr-nitrides such as CrN₂precipitate at the grain boundaries and so called sensitization occursdue to a chromium depletion layer, in which Cr content is less than inthe base metal, being formed at the grain boundaries, which results in adecrease in corrosion resistance at the grain boundaries.

In order to prevent a decrease in corrosion resistance in a weld zonedescribed above, a method is used in which the formation of Cr-carbidesand Cr-nitrides are prevented by reducing C and N in steel, adding anappropriate amount of Ti, and fixing C and N as a titanium carbonitride.By this method, the weld zone, which is formed by performing TIG weldingon sheets of the same ferritic stainless steel according to PatentLiterature 1, has good corrosion resistance.

However, since SUS304 or the like has a C content of 0.04% to 0.05%which is much more than that of this ferritic stainless steel sheetwhose C content is about 0.01%, in order to similarly preventsensitization by adding Ti in the case where this ferritic stainlesssteel sheet is welded together with a high carbon stainless steel suchas SUS304, it is necessary to increase Ti content to about 1.0%.

However, in the case where the Ti content of ferritic stainless steel isincreased to about 1.0%, there is a case where Ti and N in molten steelreact with each other to form and precipitate TiN in the course ofsolidification. This TiN has low ductility at a high temperature andcauses surface defects in hot rolling, which results in a decrease insurface quality. Since the defects formed as described above are toodeep to be eliminated during annealing of a hot-rolled sheet and acidpickling and further during cold rolling, annealing of a cold-rolledsheet and acid pickling, the defects become surface defects calledstringers caused by titanium nitrides, unless performing a treatmentsuch as surface grinding in which a thick layer is shaved off thesurface of the hot-rolled, annealed and pickled steel sheet, whichresults in a significant decrease in the surface quality of thecold-rolled, annealed and pickled steel sheet.

In addition, it is generally recommended to perform TIG welding underconditions in which the face and back sides of a steel sheet areshielded with an inert gas so that thin oxide layers called temper colorare not formed at the weld zone. However, in a practical process, thisgas shield is not sufficiently effective, and there is also a problem inthat sensitization described above is promoted by N in the air beingmixed in.

In addition, there is also a problem in that adding expensive Ti in alarge amount decreases the advantage of the steel grade which does notuse expensive Ni.

The present invention has been completed in view of the situationdescribed above, and the present invention aims to provide a ferriticstainless steel with excellent corrosion resistance in a weld zone whichis obtained even when the ferritic stainless steel is welded togetherwith an austenitic stainless steel and to provide a ferritic stainlesssteel with excellent surface quality.

The present inventors, in order to solve the problems described above,conducted close examinations and investigations regarding the influenceof the chemical composition of steel on the corrosion resistance of abase metal and a weld zone and on the occurrence behavior of surfacedefects (a scab, a pin hole, a linear scab, a shape defect such asstringers caused by titanium nitrides, and a color defect of a whitestripe type) of a steel sheet, and, as a result, obtained the followingknowledge.

(1) Sensitization can be prevented to some extent by making themicrostructure of a weld zone, which is formed by welding a ferriticstainless steel and an austenitic stainless steel, a martensite phase,in which the solid solubility limits of C and N are large, by adjustingthe contents of chemical elements promoting the formation of a ferriticphase, which are so-called ferritic former elements.

(2) When adding a very small amount of Nb, the precipitates of Nb incombination with N are formed at a temperature higher than a temperatureat which Ti nitrides are precipitated and these precipitates becomenucleation sites of Ti carbonitrides in a cooling process thereafter toform carbonitrides in combination with Ti, which results in an effect ofpreventing sensitization by Ti.

(3) There is little side effect which increases the crystallizationtemperature of a steel sheet in the case where a very small amount of Nbis added, and an inexpensive rapid annealing and pickling method using aline for manufacturing carbon steel such as that disclosed in PatentLiterature 2 can be applied.

(4) Even if N in air is mixed in a weld zone due to an incomplete gasshield, sensitization can be prevented due to the formation of AlN inthe weld zone by adding Al. Sensitization can also be prevented due tothe formation of the compounds of Sb and N in a weld zone by adding Sb.

(5) A stringer flaw caused by titanium nitrides is mainly caused by TiNwhich grows at the columnar grain boundaries, because TiN which ispresent in the part of columnar grains grows to a large size. A stringerflaw caused by titanium nitrides is hardly caused by TiN in other partsthan that in the part of columnar grains, because it is highly probablethat this kind of TiN is removed in normal post-processes such as acidpickling of a hot-rolled sheet and acid pickling of a cold-rolled sheet.

(6) In the case where Si content is large, since there is a decrease inthe solubility product of Ti and N in molten steel, the precipitation ofTi carbonitrides is promoted at a temperature higher than that at whichcolumnar grains are formed. As a result, since there is a decrease inthe amount of N in steel, it is difficult for TiN to precipitate at thecolumnar grain boundaries when columnar grains grow afterwards.Therefore, in the case where Si content is large, even if Ti content islarge to some extent, the precipitation of TiN at the columnar grainboundaries, which causes a stringer flaw caused by titanium nitrides,can be suppressed.

From the findings described above, the knowledge was obtained that aferritic stainless steel, which has an excellent corrosion resistance ina weld zone, which does not need surface grinding at the stage of ahot-rolled, annealed and pickled steel sheet, which makes a cold-rolled,annealed and pickled steel sheet having excellent surface quality, andwhich is less expensive compared to austenitic stainless steelscontaining Ni, can be achieved.

The present invention has been completed on the basis of the knowledgedescribed above and the subject matter of the present invention includesthe following.

[1] Ferritic stainless steel with excellent corrosion resistance in aweld zone and surface quality, the steel having a chemical compositioncontaining, by mass %, C: 0.003% or more and 0.012% or less, Si: 0.30%or more and 0.60% or less, Mn: 0.10% or more and 0.35% or less, P:0.040% or less, S: 0.020% or less, Cr: 17.0% or more and 19.0% or less,Ni: more than 0.10% and 0.30% or less, Ti: 0.10% or more and 0.40% orless, Nb: 0.005% or more and less than 0.050%, Mo: 0.20% or less, N:0.005% or more and 0.015% or less, Cu: 0.3% or more and 0.5% or less,Mg: less than 0.0005%, and the balance being Fe and inevitableimpurities.

[2] The ferritic stainless steel with excellent corrosion resistance ina weld zone and surface quality according to item [1], the steel havingthe chemical composition further containing, by mass %, Al: 0.01% ormore and 0.5% or less.

[3] The ferritic stainless steel with excellent corrosion resistance ina weld zone and surface quality according to item [1] or [2], the steelhaving the chemical composition further containing, by mass %, Sb: 0.05%or more and 0.30% or less.

[4] The ferritic stainless steel with excellent corrosion resistance ina weld zone and surface quality according to any one of items [1] to[3], the steel having the chemical composition further containing, bymass %, one or two of Zr: 0.01% or more and 0.60% or less and V: 0.01%or more and 0.50% or less.

The ferritic stainless steel according to the present invention can besuitably used as materials for kitchen instruments, architecturalinteriors, industrial machineries, and automobile parts, because theferritic stainless steel has excellent corrosion resistance in a weldzone and surface quality when the ferritic stainless steel is weldedeven together with an austenitic stainless steel.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The factors constituting the present invention will be describedhereafter.

1. Regarding chemical composition

First, the reason for the preferred chemical composition of the steelaccording to the present invention will be described. Here, % used whendescribing a chemical composition always means mass %.

C: 0.003% or more and 0.012% or less

It is preferable that C content be as small as possible, because C tendsto combine with Cr to form Cr carbides, and because intergranularcorrosion is caused by Cr carbides which are formed in a heat affectedzone when welding is performed. Therefore, the C content is set to be0.012% or less. On the other hand, since a long time is necessary forsmelting in the case where the C content is excessively small, the Ccontent is set to be 0.003% or more and 0.012% or less, preferably0.003% or more and 0.010% or less.

Si: 0.30% or more and 0.60% or less

Si is a chemical element which is important in the present invention. Inthe case where Si is added in an appropriate mount, there is a decreasein the solubility product of Ti and N and therefore the precipitation ofTi carbonitrides is promoted at a temperature higher than that at whichcolumnar grains are formed. As a result, since there is a decrease inthe amount of N in steel, it is difficult for TiN to precipitate at thecolumnar grain boundaries when columnar grains grow afterwards. In thecase of the chemical composition according to the present invention, byadding Si in an amount of 0.30% or more, it is possible to suppress theprecipitate of TiN at the columnar grain boundaries, which causes astringer flaw caused by titanium nitrides. However, in the case wherethe Si content is more than 0.60%, there is a decrease in productivity,because there is a decrease in the pickling performance of a cold-rolledsheet in a rapid pickling method which is used in an apparatus formanufacturing ordinary steel described above. In addition, in the casewhere the Si content is excessively large, there is a decrease informability due to the excessive hardness of the material. Therefore,the Si content is set to be 0.30% or more and 0.60% or less, preferably0.40% or more and 0.50% or less.

Mn: 0.10% or more and 0.35% or less Since Mn is effective fordeoxidation, the Mn content is set to be 0.10% or more. In addition,since Mn is a chemical element promoting the formation of an austenitephase (an austenite former element), Mn promotes the formation of amartensite phase in a weld zone which is formed when a ferriticstainless steel is welded together with an austenitic stainless steel(hereinafter, called the weld zone of steels of different grades).However, in the case where the Mn content is excessively large, Mncombines with S in the steel to form MnS which is a soluble sulfide,which results in a decrease in corrosion resistance. Therefore, the Mncontent is set to be 0.10% or more and 0.35% or less, preferably 0.10%or more and 0.25% or less.

P: 0.040% or less

P is a chemical element which has a negative effect on corrosionresistance, and, moreover, which decreases hot formability. Inparticular, these tendencies become significant in the case where the Pcontent is more than 0.040%. Therefore, the P content is set to be0.040% or less, preferably 0.030% or less.

S: 0.020% or less

S is a chemical element which has a negative effect on corrosionresistance. In particular, in the case where S is present together withMn, S becomes a source of pitting as a result of forming MnS, whichresults in a decrease in corrosion resistance. This negative effectbecomes significant in the case where S content is more than 0.020%.Therefore, the S content is set to be 0.020% or less, preferably 0.010%or less, more preferably 0.006% or less.

Cr: 17.0% or more and 19.0% or less

Cr is a chemical element which is essential for increasing the corrosionresistance of a base metal by forming a passivation film on the surfaceof a stainless steel. Although Cr content of 17.0% or more is necessaryin order to achieve good corrosion resistance, in the case where the Crcontent is more than 19.0%, a decrease in corrosion resistance cannot beprevented in the weld zone of steels of different grades with SUS304,because a martensite phase cannot be formed in the weld zone. Therefore,the Cr content is set to be 17.0% or more and 19.0% or less, preferably17.5% or more and 18.5% or less.

Ni: more than 0.10% and 0.30% or less

Ni is a chemical element which contributes to improving crevicecorrosion resistance. Moreover, since Ni is a chemical element promotingthe formation of an austenite phase (an austenite former element) likeMn, Ni promotes the formation of a martensite phase in the weld zone ofsteels of different grades. However, there is an increase in SCCsensitivity in the case where Ni content is more than 0.30%, and Ni isan expensive chemical element. Therefore, the Ni content is set to bemore than 0.10% and 0.30% or less, preferably 0.20% or more and 0.30% orless.

Nb: 0.005% or more and less than 0.050%

The addition of a small amount of Nb is also one of the importantfactors for the present invention. Nb forms carbonitrides more than Cror Ti. In particular, in the case of the weld zone of steels ofdifferent grades, in weld metal and a heat affected zone, the formationof Nb carbonitrides begins at a temperature higher than the temperatureat which Ti carbonitrides are formed. Although the reason is not clear,in a cooling process thereafter, a small amount of Nb carbonitridesbecomes nucleation sites of Ti carbonitrides. That is to say, theaddition of a small amount of Nb promotes the formation of Ticarbonitrides. Therefore, the capability of Ti for fixing C and N inweld metal and a heat affected zone becomes stronger than that in thecase where Nb is not contained. Therefore, the Nb content is set to be0.005% or more.

On the other hand, in the case where the Nb content is 0.050% or more,since there is an increase in the recrystallization temperature of acold-rolled steel sheet, it is necessary to anneal the steel sheet at atemperature higher than that in the case where Nb is not added in orderto achieve good mechanical properties. Therefore, there is an increasein the thickness of an oxide layer, which is formed when finishingannealing is performed, in comparison to that in the case where Nb isnot added. Therefore, there is a decrease in productivity due to adecrease in the pickling performance of a cold-rolled steel sheet in arapid pickling method which is used in an apparatus for manufacturingcarbon steel described above. Therefore, the Nb content is set to be0.005% or more and less than 0.050%, preferably 0.01% or more and lessthan 0.050%.

Ti: 0.10% or more and 0.40% or less

It is necessary to add Ti in an amount of 0.10% or more in order toachieve good corrosion resistance in the weld zone of steels ofdifferent grades which is formed when a ferritic stainless steel iswelded together with an austenitic stainless steel (in order to preventsensitization). However, in the case where the Ti content is more than0.40%, since there is an increase in the amount of stringer flaws causedby titanium nitrides due to the formation of TiN at the columnar grainboundaries, it is necessary to grind the surface of a hot-rolled,annealed and pickled steel sheet by using a grinder in order to achievegood surface quality. Therefore, the Ti content is set to be 0.10% ormore and 0.40% or less, preferably 0.20% or more and 0.40% or less.

Mo: 0.20% or less

Mo is a chemical element which strengthens a passivation film andsignificantly increases corrosion resistance, and these effects arerealized in the case where the Mo content is 0.01% or more. However,since Mo is a chemical element promoting the formation of a ferritephase, which is a so-called ferrite former element, a martensite phaseis not formed in the weld zone of steels of different grades which isformed when a ferritic stainless steel is welded together with anaustenitic stainless steel, in the case where the Mo content is morethan 0.20%, and a ferrite phase containing a small amount of C and N inthe form of solid solution is formed, which results in sensitization notbeing prevented. Therefore, the Mo content is set to be 0.20% or less.In addition, since Mo causes a decrease in the toughness of a hot-rolledsteel sheet, it is preferable that the Mo content be 0.10% or less.

N: 0.005% or more and 0.015% or less

N tends to combine with Cr to form Cr nitrides. It is preferable thatthe N content be as small as possible, because Cr nitrides causeintergranular corrosion in the case where Cr nitrides are formed in theweld zone of steels of different grades and in a heat affected zone whenwelding is performed. In addition, it is preferable that N content be assmall as possible, because N is a chemical element which causes stringerflaws caused by titanium nitrides. However, since smelting takes a longtime in the case where the N content is excessively small, the N contentis set to be 0.005% or more and 0.015% or less, preferably 0.005% ormore and 0.012% or less.

Cu: 0.3% or more and 0.5% or less

Cu is a chemical element which increases corrosion resistance, inparticular in the case where a steel sheet is placed in an aqueoussolution or covered with weakly acidic water drops. This is presumed tobe because Cu suppresses the dissolution of the base metal of the steelsheet by first dissolving at a certain electrochemical potential in theaqueous solution or the water drops and then by covering the surface ofthe base metal. On the other hand, in the case where Cu content is morethan 0.5%, there is a decrease in hot formability and surface defectsare caused by the formation of an oxide having a low meltingtemperature, which is called red scale, on the surface of a hot-rolledslab due to Cu when hot rolling is performed. Therefore, the Cu contentis set to be 0.3% or more and 0.5% or less, preferably 0.3% or more and0.4% or less.

Mg: less than 0.0005%

Mg is an impurity which is mixed in mainly from the bricks of aconverter furnace. Since Mg becomes the source of various kinds ofinclusion and nucleation sites of other kinds of inclusion even if theamount of Mg which is mixed in is small, and since Mg is less likely toreform into a solid solution even when a treatment such as annealing isperformed, Mg decreases the surface quality of a hot-rolled, annealedand pickled steel sheet and a cold-rolled, annealed and pickled steelsheet. Therefore, the Mg content is set to be less than 0.0005%,preferably less than 0.0003%.

The basic chemical composition according to embodiments of the presentinvention is as described above and the balance consists of Fe andinevitable impurities. Al and Sb may be further added as selectivechemical elements from the viewpoint of forming a gas shield for TIGwelding and preventing the sensitization of the weld zone of steels ofdifferent grades. Moreover, Zr and V may be added as selective chemicalelements in order to improve the corrosion resistance of the weld zoneof steels of different grades. Here, as an example of inevitableimpurities, Ca: 0.0020% or less is acceptable.

Al: 0.01% or more and 0.5% or less

The addition of Al is necessary in the case where a gas shield for TIGwelding is not sufficiently effective. Generally, the face and backsides of a steel sheet are shielded with gas when TIG welding isperformed as described above. However, in the case where the shape ofthe weld zone of steels of different grades is complicated, since thegas shield is not sufficiently effective, N in air may be mixed in theweld metal, so that the amount of N is more than the solid solubilitylimit of a martensite phase, which results in such a case wheresensitization is not completely prevented only by Ti which is added inadvance. In this case, it is effective to add Al in advance in order toprevent sensitization. This is because Al fixes N which is mixed in theweld metal and forms AlN as a result of combining with N. This effectcan be realized in the case where the Al content is 0.01% or more.However, in the case where the Al content is excessively large,non-metal inclusions are formed at the slab stage, which results in adecrease in surface quality, and which results in a decrease in thetoughness of a hot-rolled sheet. Therefore, in the case where Al isadded, it is preferable that the Al content be 0.01% or more and 0.5% orless, preferably 0.1% or more and 0.5% or less, more preferably 0.15% ormore and 0.25% or less.

Sb: 0.05% or more and 0.30% or less

It is better to add Sb in the case where a component has a complicatedshape, because Sb is a chemical element that is, like Al, effective forfixing N which is mixed in from air in the case where a gas shield forTIG welding is not sufficiently effective. However, in the case where Sbcontent is excessively large, non-metal inclusions are formed at theslab stage, which results in a decrease in the surface quality, andwhich results in a decrease in the toughness of a hot-rolled sheet.Therefore, in the case where Sb is added, it is preferable that the Sbcontent be 0.05% or more and 0.30% or less, more preferably 0.05% ormore and 0.15% or less.

Zr: 0.01% or more and 0.60% or less

Zr is a chemical element which is preferably added in consideration ofthe corrosion resistance of a weld zone, because Zr increases corrosionresistance in the weld zone of steels of the same grade and in the weldzone of steels of different grades by forming, like Ti, carbonitridesmore preferentially than Cr. However, Zr is more expensive than Ti andforms intermetallic compounds in the case where Zr content isexcessively large, which results in a decrease in the toughness of ahot-rolled sheet. Therefore, in the case where Zr is added, it ispreferable that the Zr content be 0.01% or more and 0.60% or less, morepreferably 0.1% or more and 0.35% or less.

V: 0.01% or more and 0.50% or less

V is also a chemical element which is effective for increasing corrosionresistance in the weld zone of steels of the same grade and in the weldzone of steels of different grades by forming, like Ti, carbonitridesmore preferentially than Cr. This effect can be realized in the casewhere the V content is 0.01% or more. However, in the case where the Vcontent is more than 0.50%, there is a decrease in mechanicalproperties. Therefore, in the case where V is contained, it ispreferable that the V content be 0.01% or more and 0.50% or less, morepreferably 0.02% or more and 0.05% or less.

2. Regarding manufacturing conditions

Subsequently, a suitable method for manufacturing the steel according tothe present invention will be described. Steel having the chemicalcomposition described above is smelted using a well-known method such asone using a converter furnace, an electric furnace, or a vacuum meltingfurnace, and the steel is made into a steel material (slab) using acontinuous casting method or an ingot casting and slabbing process. Thissteel material is heated at a temperature of 1100° C. to 1250° C. for aduration of 1 to 24 hours and hot-rolled into a hot-rolled sheet, orhot-rolled into a hot-rolled sheet without undergoing such heating.

The hot-rolled steel sheet is normally subjected to annealing of ahot-rolled sheet at a temperature of 800° C. to 1100° C. for a durationof 1 to 10 minutes. However, annealing of a hot-rolled sheet may beomitted depending on use application. Then, after being subjected topickling of a hot-rolled sheet, the hot-rolled sheet is cold-rolled intoa cold-rolled sheet, and the cold-rolled sheet is made into a productsheet by performing finishing annealing. It is preferable that coldrolling be performed with a rolling reduction ratio in thickness of coldrolling of 50% or more in order to achieve good ductility, bendingperformance, and press forming performance and in order to perform shapeleveling. It is preferable that recrystallization annealing of acold-rolled steel sheet be performed at a temperature of 800° C. to 950°C. in the case of No. 2B finish in order to achieve good mechanicalproperties and pickling performance.

However, in the case of a functional product, it is most preferable thatthe product be manufactured in an inexpensive process using a rapidpickling method which uses an annealing and pickling line for carbonsteel which utilizes a line for manufacturing carbon steel as describedabove, and, in this case, it is most preferable that the annealingtemperature be 800° C. to 900° C.

In addition, in the case of a material for a part of which luster isrequired more than usual, it is effective to perform finishing annealingusing bright annealing. In addition, as described above, a treatmentsuch as polishing may be performed after cold rolling and machining havebeen performed in order to achieve further better surface quality,although there is a disadvantage in cost.

EXAMPLE 1

Ferritic stainless steels having chemical compositions of examples No. 1through No. 8 and comparative examples No. 9 through No. 12 given inTable 1 were smelted using a small vacuum melting furnace having acapacity of 50 kg. These ingots were heated at a temperature of 1150° C.in a furnace under an Ar gas purge and hot-rolled into hot-rolled sheetshaving a thickness of 3.5 mm.

Subsequently, these hot-rolled sheets were subjected to annealing of ahot-rolled sheet in air at a temperature of 950° C. for a duration of 1minute, and a surface treatment using shot blasting with glass beads.Then, descaling was performed using a pickling process in which thesteel sheets were dipped in a sulfuric acid solution containing sulfuricacid at a concentration of 20 mass % at a temperature of 80° C. for aduration of 120 seconds and then in a mixed acid solution containingnitric acid at a concentration of 15 mass % and hydrofluoric acid at aconcentration of 3 mass % at a temperature of 55° C. for a duration of60 seconds.

Moreover, after the descaling was performed, the hot-rolled sheets werecold-rolled into cold-rolled steel sheets having a thickness of 1.0 mm,subjected to annealing in a furnace in air at a temperature of 900° C.for a duration of 1 minute in order to obtain cold-rolled and annealedsheets. These cold-rolled and annealed sheets were subjected toelectrolytic descaling three times in a solution containing Na₂SO₄ at aconcentration of 20 mass % at a temperature of 80° C. with a current of3 Ampere/dm² for a duration of 10 seconds and descaling in a mixed acidsolution containing nitric acid at a concentration of 5 mass % andhydrofluoric acid at a concentration of 3 mass % at a temperature of 55°C. for a duration of 30 seconds in order to obtain cold-rolled, annealedand pickled sheets.

TABLE 1 Chemical Composition (mass %) No. C Si Mn P S Cr Ni Cu Ti Nb MoN Mg Al Sb Zr V Note 1 0.010 0.38 0.21 0.025 0.004 18.5 0.19 0.35 0.220.02 0.05 0.009 0.0002 — — — — Exam- 2 0.004 0.45 0.15 0.023 0.007 17.60.12 0.38 0.32 0.03 0.02 0.006 0.0003 — — — — ple 3 0.012 0.50 0.200.020 0.002 18.6 0.21 0.42 0.28 0.04 0.07 0.012 0.0002 — — — — 4 0.0070.55 0.18 0.021 0.003 17.3 0.25 0.45 0.18 0.04 0.08 0.010 0.0004 — — — —5 0.003 0.32 0.13 0.023 0.005 18.2 0.18 0.39 0.29 0.02 0.10 0.006 0.00020.16 — — — 6 0.008 0.39 0.15 0.025 0.002 17.9 0.15 0.33 0.34 0.01 0.090.012 0.0003 0.25 — — 0.1 7 0.009 0.55 0.25 0.019 0.003 18.2 0.27 0.500.25 0.03 0.09 0.011 0.0003 — 0.10 0.10 — 8 0.006 0.41 0.29 0.028 0.00217.3 0.26 0.33 0.15 0.02 0.06 0.007 0.0002 0.11 0.05 — — 9 0.010 0.430.20 0.027 0.006 16.1 0.16 0.42 0.28 0.02 0.11 0.009 0.0004 — — — — Com-10 0.004 0.42 0.23 0.028 0.001 19.5 0.24 0.30 0.35 0.02 0.15 0.0070.0002 0.14 — — — parative 11 0.011 0.44 0.22 0.021 0.002 18.1 0.22 0.340.08 0.02 0.12 0.014 0.0003 — — 0.05 — Exam- 12 0.005 0.25 0.24 0.0180.003 18.3 0.16 0.33 0.29 0.06 0.09 0.008 0.0002 0.20 — — 0.2 pleNotation: Underlined value is out of range according to the presentinvention.

Using these cold-rolled, annealed and pickled sheets, corrosionresistance was evaluated by performing a salt spray cyclic corrosiontest(hereafter, CCT, at times). Subsequently, these cold-rolled,annealed and pickled sheets were polished to #600 finish and used forcorrosion resistance evaluation by performing a CCT. Subsequently, usingeach of these polished sheets, a corrosion test on the weld zone ofsteels of the same grade formed by performing TIG welding was carriedout. In this test, two pieces of sheets which were cut out of eachsample material were welded by performing TIG welding, and the surfaceof the welded material was polished to #600 finish in order toinvestigate corrosion resistance by performing a CCT. The conditions ofTIG welding will be described below. Welding current was controlled sothat the width of the back bead was 3 mm or more. The surface on theback bead side was evaluated.

Welding potential: 10 Volts

Welding current: from 90 Ampere to 110 Ampere

Welding speed: 600 mm/min

Electrode: tungsten electrode of 1.6 mm

Shield gas: face bead side Ar 20 Liter/min, back bead side Ar 20Liter/min

Moreover, using the same sample materials, a corrosion test on the weldzone of steels of different grades with SUS304 was carried out. In thistest, a piece of sheet which was cut out of each sample material waswelded with a sheet of SUS304 having a thickness of 1.0 mm by performingTIG welding, and the surface of the welded material was polished to #600finish in order to investigate corrosion resistance by performing a CCT.The conditions of TIG welding were almost the same as those used forsteels of the same grade described above.

In the salt spray cyclic corrosion test, a cycle in which spraying 5%NaCl aqueous solution at 35° C., 2 hrs, →drying at 60° C., 4 hrs,relative humidity: 20% to 30%, and →wetting at 40° C., 2 hrs, relativehumidity: 95% or more were performed in this order was repeated for 15cycles. The obtained results are given in Table 2.

TABLE 2 Corrosion Corrosion Corrosion Resistance Resistance SurfaceResistance Corrosion by CCT of by CCT of Quality of by CCT of ResistanceWeld Zone Weld Zone Cold-Rolled, Cold-Rolled, by CCT of of Steels ofSteels of Annealed and Annealed and Sheet Polished of Same Different No.Pickled Sheet Pickled Sheet to #600 Finish Grade Grades Others Note 1 ⊙⊙ ⊙ ⊙ ⊙ — Example 2 ⊙ ⊙ ⊙ ⊙ ⊙ — 3 ⊙ ⊙ ⊙ ⊙ ⊙ — 4 ⊙ ⊙ ⊙ ⊙ ⊙ — 5 ⊙ ⊙ ⊙ ⊙ ⊙— 6 ⊙ ⊙ ⊙ ⊙ ⊙ — 7 ⊙ ⊙ ⊙ ⊙ ⊙ — 8 ⊙ ⊙ ⊙ ⊙ ⊙ — 9 ⊙ X X X X — Comparative 10⊙ ⊙ ◯ ◯ X — Example 11 ⊙ ◯ ◯ X X — 12 Δ Δ ⊙ ⊙ ◯ Residual Scale,Decreased Ductility

Here, the judging criteria of the tests given in Table 2 will bedescribed hereafter.

(1) Surface appearance after cold rolling, pickling and annealing hadbeen performed: judging on the basis of the ratio of the length of aportion in which surface defects were found to the total length of thesample, ⊙ represents the case where a defect ratio is less than 5%(satisfactory: very good), ◯ represents the case where a defect ratio is5% or more and less than 10% (satisfactory: good), Δ represents the casewhere a defect ratio is 10% or more and less than 20% (unsatisfactory)and x represents the case where a defect ratio is 20% or more(unsatisfactory: very bad).

(2) The results of the CCT on cold-rolled, annealed and pickled sheetsand the sheets polished to #600 finish: judging on the basis of an areain which rust occurred after 15 cycles of the test, ⊙ represents thecase where a rust area ratio is less than 10% (satisfactory: very good),◯ represents the case where a rust area ratio is 10% or more and lessthan 20% (satisfactory: good), Δ represents the case where a rust arearatio is 20% or more and less than 30% (unsatisfactory) and x representsthe case where a rust area ratio is 30% or more (unsatisfactory: verybad).

(3) The results of the test of the corrosion resistance on the weld zoneof steels of the same grade: judging on the basis of a rust area ratioafter 15 cycles of the CCT which was performed on the samples obtainedby performing TIG butt welding on steels of the same grade and byeliminating the temper color of the weld zone with #600 emery paper, ⊙represents the case where a rust area ratio is less than 10%(satisfactory: very good), ◯ represents the case where a rust area ratiois 10% or more and less than 20% (satisfactory: good), Δ represents thecase where a rust area ratio is 20% or more and less than 30%(unsatisfactory) and x represents the case where a rust area ratio is30% or more (unsatisfactory: very bad).

(4) The results of the test of the corrosion resistance on the weld zoneof steels of different grades with SUS304: judging on the basis of arust area ratio after 15 cycles of the CCT which was performed on thesamples obtained by performing TIG butt welding on each sample materialand SUS304 and by eliminating the temper color of the weld zone with#600 emery paper, ⊙ represents the case where a rust area ratio is lessthan 10% (satisfactory: very good), ◯ represents the case where a rustarea ratio is 10% or more and less than 20% (satisfactory: good), Δrepresents the case where a rust area ratio is 20% or more and less than30% (unsatisfactory) and x represents the case where a rust area ratiois 30% or more (unsatisfactory: very bad).

The steels having chemical compositions No. 1 through No. 8 which werewithin the range according to preferred embodiments of the presentinvention were excellent in terms of corrosion resistance and surfacequality in the cold-rolled, annealed and pickled sheet state, in thepolished sheet state, in the weld zone of steels of the same grade andin the weld zone of the steels of different grades with SUS304.

On the other hand, comparative example No. 9 which had a small Crcontent of 16.1% had a large rust area, which means that No. 9 was poorin terms of corrosion resistance.

In addition, comparative example No. 10 which had a large Cr content of19.5% had a large rust area in the weld zone of steels of differentgrades, which means that No. 10 was poor in terms of corrosionresistance. It is thought to be because a martensite phase was notformed in the weld zone of steels of different grades due to the highcontent of Cr which is a ferrite former element.

In addition, comparative example No. 11 which had a small Ti content of0.08% had large rust areas in the weld zone of steels of the same gradeand in the weld zone of steels of different grades, which means that No.11 was poor in terms of corrosion resistance.

Moreover, in the case of comparative example No. 12 which had Nb contentwhich was more than the range according to preferred embodiments of thepresent invention, some scale residual was found on the surface of thebase metal, which means that No. 12 was poor in terms of corrosionresistance after annealing of a cold-rolled sheet had been performed.

EXAMPLE 2

Subsequently, steels having chemical compositions of examples No. 13through No. 18 and comparative examples No. 19 through No. 22 given inTable 3 were smelted using a VOD (Vacuum Oxygen Decarburization) havinga capacity of 150 ton and cast into slabs by performing continuouscasting. These slabs were heated at a temperature of 1150° C. andhot-rolled into hot-rolled steel sheets in a coiled state having athickness of 3.5 mm. Then, these hot-rolled sheets were subjected toannealing in an atmosphere of a coke oven gas having an air ratio of 1.3at a temperature of 950° C. for a duration of 1 to 5 minutes, shotblasting on the surface with iron beads, descaling by pickling in whichthe steel sheets were dipped in a sulfuric acid solution containingsulfuric acid at a concentration of 20 mass % at a temperature of 80° C.for a duration of 120 seconds and then in a mixed acid solutioncontaining nitric acid at a concentration of 15 mass % and hydrofluoricacid at a concentration of 3 mass % at a temperature of 55° C. for aduration of 60 seconds in order to obtain hot-rolled annealed andpickled sheets in the coiled state.

Moreover, the hot-rolled sheets were cold-rolled into cold-rolled sheetshaving a thickness of 1.0 mm, subjected to annealing in a furnace in anatmosphere of a coke oven gas having an air ratio of 1.3 at atemperature of 900° C. for a duration of 2 minutes, electrolyticdescaling three times in a solution containing Na₂SO₄ at a concentrationof 20 mass % at a temperature of 80° C. with a current of 3 Ampere /dm²for a duration of 10 seconds and descaling in a mixed acid solutioncontaining nitric acid at a concentration of 5 mass % and hydrofluoricacid at a concentration of 3 mass % at a temperature of 55° C. for aduration of 30 seconds in order to obtain cold-rolled, annealed andpickled steel sheets.

At this stage, the surface quality of the cold-rolled, annealed andpickled steel sheets obtained as described above was evaluated by avisual test.

TABLE 3 Chemical Composition (mass %) No. C Si Mn P S Cr Ni Cu Ti Nb MoN Mg Al Sb Zr V Note 13 0.009 0.40 0.22 0.027 0.003 18.7 0.16 0.37 0.230.01 0.07 0.008 0.0003 — — — — Example 14 0.012 0.37 0.18 0.022 0.00817.4 0.22 0.33 0.29 0.02 0.05 0.013 0.0004 — — — — 15 0.005 0.39 0.160.021 0.009 17.8 0.20 0.43 0.19 0.01 0.11 0.007 0.0002 — — — — 16 0.0050.55 0.24 0.020 0.005 18.3 0.19 0.35 0.25 0.02 0.04 0.005 0.0002 — — — —17 0.010 0.49 0.20 0.017 0.006 18.6 0.22 0.31 0.35 0.02 0.11 0.0130.0001 0.23 — — 0.2 18 0.004 0.39 0.15 0.025 0.008 18.2 0.22 0.32 0.330.03 0.15 0.006 0.0003 — 0.15 0.11 — 19 0.008 0.41 0.18 0.022 0.006 18.30.20 0.35 0.25 0.02 0.35 0.009 0.0005 — — — — Comparative Ex- 20 0.0040.42 0.23 0.028 0.001 19.7 0.24 0.30 0.35 0.02 0.15 0.007 0.0002 — — — —ample 21 0.011 0.10 0.21 0.020 0.003 17.9 0.21 0.36 0.33  0.004 0.080.013 0.0001 — — — — 22 0.005 0.22 0.24 0.018 0.003 18.3 0.16 0.33 0.290.01 0.09 0.008 0.0008 0.20 — — 0.2 Notation: Underlined value is out ofrange according to the present invention.

Using these cold-rolled, annealed and pickled sheets, corrosionresistance was evaluated by performing a CCT.

Subsequently, these cold-rolled, annealed and pickled sheets werepolished to #600 finish and used for corrosion resistance evaluation, acorrosion resistance test on a weld zone and a corrosion resistance teston the weld zone of steels of different grades with SUS304.

The results obtained as described above are given in Table 4.

TABLE 4 Corrosion Corrosion Corrosion Resistance Resistance SurfaceResistance Corrosion by CCT of by CCT of Quality of by CCT of ResistanceWeld Zone Weld Zone Cold-Rolled, Cold-Rolled, by CCT of of Steels ofSteels of Annealed and Annealed and Sheet Polished of Same Different No.Pickled Sheet Pickled Sheet to #600 Finish Grade Grades Others Note 13 ⊙⊙ ⊙ ⊙ ⊙ — Example 14 ◯ ⊙ ⊙ ⊙ ⊙ — 15 ⊙ ⊙ ⊙ ⊙ ⊙ — 16 ⊙ ⊙ ⊙ ⊙ ⊙ — 17 ◯ ⊙ ⊙⊙ ⊙ — 18 ⊙ ⊙ ⊙ ⊙ ⊙ — 19 ⊙ ⊙ ⊙ ⊙ X — Comparative 20 ⊙ ⊙ ⊙ ⊙ X — Example21 X ⊙ ⊙ ⊙ Δ — 22 X ◯ ◯ ◯ ◯ —

Here, the judging criteria of the tests given in Table 4 will bedescribed hereafter.

(1) Surface appearance after cold rolling, pickling and annealing hadbeen performed: judging on the basis of the ratio of the length of aportion in which surface defects were found to the total length of thesample, ⊙ represents the case where a defect ratio is less than 5%(satisfactory: very good), ◯ represents the case where a defect ratio is5% or more and less than 10% (satisfactory: good), Δ represents the casewhere a defect ratio is 10% or more and less than 20% (unsatisfactory)and x represents the case where a defect ratio is 20% or more(unsatisfactory: very bad).

(2) The results of the CCT on cold-rolled, annealed and pickled sheetsand the sheets polished to #600 finish: judging on the basis of an areain which rust occurred after 15 cycles of the test, ⊙ represents thecase where a rust area ratio is less than 10% (satisfactory: very good),◯ represents the case where a rust area ratio is 10% or more and lessthan 20% (satisfactory: good), Δ represents the case where a rust arearatio is 20% or more and less than 30% (unsatisfactory) and x representsthe case where a rust area ratio is 30% or more (unsatisfactory: verybad).

(3) The results of the test of the corrosion resistance on the weld zoneof steels of the same grade: judging on the basis of a rust area ratioafter 15 cycles of the CCT which was performed on the samples obtainedby performing TIG butt welding on steels of the same grade and byeliminating the temper color of the weld zone with #600 emery paper, ⊙represents the case where a rust area ratio is less than 10%(satisfactory: very good), ◯ represents the case where a rust area ratiois 10% or more and less than 20% (satisfactory: good), Δ represents thecase where a rust area ratio is 20% or more and less than 30%(unsatisfactory) and x represents the case where a rust area ratio is30% or more (unsatisfactory: very bad).

(4) The results of the test of the corrosion resistance on the weld zoneof steels of different grades with SUS304: judging on the basis of arust area ratio after 15 cycles of the CCT which was performed on thesamples obtained by performing TIG butt welding on each sample materialand SUS304 and by eliminating the temper color of the weld zone with#600 emery paper, ⊙ represents the case where a rust area ratio is lessthan 10% (satisfactory: very good), ◯ represents the case where a rustarea ratio is 10% or more and less than 20% (satisfactory: good), Δrepresents the case where a rust area ratio is 20% or more and less than30% (unsatisfactory) and x represents the case where a rust area ratiois 30% or more (unsatisfactory: very bad).

The steels having chemical compositions No. 13 to No. 18 which werewithin the range according to preferred embodiments of the presentinvention were excellent in terms of corrosion resistance and surfacequality in the cold- rolled, annealed and pickled sheet state, in thepolished sheet state, in the weld zone of steels of the same grade andin the weld zone of the steels of different grades with SUS304. On theother hand, comparative example No. 19 which had a Mo content of 0.35%which was larger than the range according to preferred embodiments ofthe present invention had a large rust area in the weld zone of steelsof different grades, which means that No. 19 was poor in terms ofcorrosion resistance.

In addition, comparative example No. 20 which had a large Cr content of19.7% had a large rust area in the weld zone of steels of differentgrades, which means that No. 20 was poor in terms of corrosionresistance. It is thought to be because a martensite phase was notformed in the weld zone of steels of different grades due to the highcontent of Cr which is a ferrite former element.

In addition, comparative example No. 21 which had Si content which wasless than the range according to preferred embodiments of the presentinvention and comparative example No. 22 which had small Si content andlarge Mg content were poor in terms of surface quality.

EXAMPLE 3

Steels having chemical compositions of examples No. 23 through No. 28,No. 33 and comparative examples No. 29 through No. 32 given in Table 5were smelted using a small vacuum melting furnace having a capacity of50 kg. These ingots were heated at a temperature of 1150° C. in afurnace under an Ar gas purge and hot-rolled into hot-rolled sheetshaving a thickness of 3.5 mm.

Then, these hot-rolled sheets were subjected to annealing of ahot-rolled sheet in air at a temperature of 950° C. for a duration of 1minute, a surface treatment using shot blasting with glass beads, anddescaling by pickling in which the steel sheets were dipped in asulfuric acid solution containing sulfuric acid at a concentration of 20mass% at a temperature of 80° C. for a duration of 120 seconds and thenin a mixed acid solution containing nitric acid at a concentration of 15mass % and hydrofluoric acid at a concentration of 3 mass % at atemperature of 55° C. for a duration of 60 seconds.

Moreover, the hot-rolled sheets were cold-rolled into cold-rolled steelsheets having a thickness of 1.0 mm, subjected to annealing in areducing atmosphere (H₂: 5 volume %, N₂: 95 volume %, dewpoint: −40° C.)at a temperature of 900° C. for a duration of 1 minute in order toobtain cold-rolled and annealed sheets. These cold-rolled and annealedsheets were subjected to descaling using electrolysis (10 Ampere /dm²for 2 seconds) two times in a solution containing nitric acid at aconcentration of 15 mass % and hydrochloric acid in an concentration of0.5 mass % at a temperature of 50° C. in order to obtain cold-rolled,annealed and pickled sheets.

TABLE 5 Chemical Composition (mass %) No. C Si Mn P S Cr Ni Cu Ti Nb MoN Mg Al Sb Zr V Note 23 0.010 0.37 0.32 0.026 0.005 18.6 0.17 0.32 0.280.02 0.12 0.012 0.0004 — — — — Example 24 0.005 0.48 0.22 0.027 0.00617.4 0.15 0.36 0.24 0.01 0.08 0.005 0.0002 — — — — 25 0.011 0.36 0.180.024 0.003 18.9 0.22 0.41 0.32 0.03 0.15 0.011 0.0003 0.18 — — — 260.009 0.54 0.22 0.022 0.004 17.2 0.21 0.32 0.21 0.02 0.18 0.010 0.0002 —0.21 0.1 — 27 0.007 0.33 0.18 0.026 0.003 18.4 0.19 0.36 0.36 0.03 0.080.006 0.0002 0.18 — — 0.4 28 0.009 0.45 0.24 0.027 0.002 18.0 0.24 0.360.19 0.01 0.05 0.005 0.0002 0.18 0.07 — 0.1 33 0.010 0.45 0.18 0.0250.003 18.2 0.18 0.41 0.23 0.02 0.05 0.009 0.0002 0.04 — — — 29 0.0080.33 0.21 0.026 0.007 16.8 0.16 0.42 0.28 0.02 0.11 0.009 0.0004 — — — —Com- 30 0.004 0.42 0.23 0.028 0.001 19.8 0.24 0.30 0.35 0.02 0.15 0.0070.0002 0.20 — — — parative 31 0.011 0.15 0.22 0.021 0.002 17.9 0.22 0.340.08 0.02 0.40 0.012 0.0003 — — — — Example 32 0.005 0.25 0.24 0.0180.003 18.3 0.16 0.33 0.29 0.10 0.09 0.008 0.0002 — — — 0.3 Notation:Underlined value is out of range according to the present invention.

Using these cold-rolled, annealed and pickled sheets, corrosionresistance was evaluated by performing a CCT. Moreover, thesecold-rolled, annealed and pickled sheets were polished to #600 finishand used for corrosion resistance evaluation by performing a CCT and acorrosion test on the weld zone of steels of the same grade formed byperforming TIG welding. In this test, two pieces of sheets which werecut out of each sample material were welded by performing TIG welding,and the surface of the welded material was polished to #600 finish inorder to investigate corrosion resistance by performing a CCT. Theconditions of TIG welding will be described below. Welding current wascontrolled so that the width of the back bead was 3 mm or more. Thesurface on the back bead side was evaluated.

Welding potential: 10 Volts

Welding current: from 90 Ampere to 110 Ampere

Welding speed: 600 mm/min

Electrode: tungsten electrode of 1.6 mm

Shield gas: face bead side Ar +20 vol % N₂ 20 Liter/min, back bead sideAr+20 vol % N₂ 20 Liter/min

Moreover, using the same sample materials, a corrosion test on the weldzone of steels of different grades with SUS304 was carried out. In thistest, a piece of sheet which was cut out of each sample material waswelded with a sheet of SUS304 having a thickness of 1.0 mm by performingTIG welding, and the surface of the welded material was polished to #600finish in order to investigate corrosion resistance by performing a CCT.The conditions of TIG welding were almost the same as those used forsteels of the same grade described above. In the salt spray cycliccorrosion test, a cycle in which spraying 5% NaCl aqueous solution at35° C., 2 hrs, →drying at 60° C., 4 hrs, relative humidity: 20% to 30%,and →wetting at 40° C., 2 hrs, relative humidity: 95% or more wereperformed in this order was repeated for 15 cycles. The obtained resultsare given in Table 6.

TABLE 6 Corrosion Corrosion Corrosion Corrosion Resistance ResistanceResistance Surface Resistance by CCT of by CCT of by CCT of Quality ofby CCT of Sheet Weld Zone Weld Zone Cold-Rolled, Cold-Rolled, Polishedof Steels of of Steels of Annealed and Annealed and to #600 SameDifferent No. Pickled Sheet Pickled Sheet Finish Grade Grades OthersNote 23 ⊙ ⊙ ⊙ ◯ ◯ — Example 24 ⊙ ⊙ ⊙ ◯ ◯ — 25 ⊙ ⊙ ⊙ ⊙ ⊙ — 26 ◯ ◯ ⊙ ⊙ ⊙ —27 ⊙ ⊙ ⊙ ⊙ ⊙ — 28 ⊙ ⊙ ⊙ ⊙ ⊙ — 33 ⊙ ⊙ ⊙ ⊙ ⊙ — 29 ⊙ X X X X — Comparative30 ⊙ ⊙ ◯ Δ X — Example 31 Δ ◯ ◯ ◯ X — 32 X X ⊙ ◯ X Residual Scale,Decreased Ductility

Here, the judging criteria of the tests given in Table 6 will bedescribed hereafter.

(1) Surface appearance after cold rolling, pickling and annealing hadbeen performed: judging on the basis of the ratio of the length of aportion in which surface defects were found to the total length of thesample, ⊙ represents the case where a defect ratio is less than 5%(satisfactory: very good), ◯ represents the case where a defect ratio is5% or more and less than 10% (satisfactory: good), Δ represents the casewhere a defect ratio is 10% or more and less than 20% (unsatisfactory)and x represents the case where a defect ratio is 20% or more(unsatisfactory: very bad).

(2) The results of the CCT on cold-rolled, annealed and pickled sheetsand the sheets polished to #600 finish: judging on the basis of an areain which rust occurred after 15 cycles of the test, ⊙ represents thecase where a rust area ratio is less than 10% (satisfactory: very good),◯ represents the case where a rust area ratio is 10% or more and lessthan 20% (satisfactory: good), Δ represents the case where a rust arearatio is 20% or more and less than 30% (unsatisfactory) and x representsthe case where a rust area ratio is 30% or more (unsatisfactory: verybad).

(3) The results of the test of the corrosion resistance on the weld zoneof steels of the same grade: judging on the basis of a rust area ratioafter 15 cycles of the CCT which was performed on the samples obtainedby performing TIG butt welding on steels of the same grade and byeliminating the temper color of the weld zone with #600 emery paper, ⊙represents the case where a rust area ratio is less than 10%(satisfactory: very good), ◯ represents the case where a rust area ratiois 10% or more and less than 20% (satisfactory: good), Δ represents thecase where a rust area ratio is 20% or more and less than 30%(unsatisfactory) and x represents the case where a rust area ratio is30% or more (unsatisfactory: very bad).

(4) The results of the test of the corrosion resistance on the weld zoneof steels of different grades with SUS304: judging on the basis of arust area ratio after 15 cycles of the CCT which was performed on thesamples obtained by performing TIG butt welding on each sample materialand SUS304 and by eliminating the temper color of the weld zone with#600 emery paper, ⊙ represents the case where a rust area ratio is lessthan 10% (satisfactory: very good), ◯ represents the case where a rustarea ratio is 10% or more and less than 20% (satisfactory: good), Δrepresents the case where a rust area ratio is 20% or more and less than30% (unsatisfactory) and x represents the case where a rust area ratiois 30% or more (unsatisfactory: very bad).

The steels having chemical compositions No. 23 to No. 28 and No. 33which were within the range according to preferred embodiments of thepresent invention were excellent in terms of corrosion resistance andsurface quality in the cold-rolled, annealed and pickled sheet state, inthe polished sheet state, in the weld zone of steels of the same gradeeven in the case of an insufficient gas shield and in the weld zone ofthe steels of different grades with SUS304 even in the case of aninsufficient gas shield. In particular, No. 25 through No. 28 and No.33, to which Al, Sb, Zr and V were added, were excellent in terms ofcorrosion resistance in the weld zone of steels of different grades withSUS304 even in the case of an insufficient gas shield.

On the other hand, comparative example No. 29 which had a small Crcontent of 16.8% had a large rust area, which means that No. 29 was poorin terms of corrosion resistance.

In addition, comparative example No. 30 which had a large Cr content of19.8% had a large rust area in the weld zone of steels of differentgrades, which means that No. 30 was poor in terms of corrosionresistance. It is thought to be because a martensite phase was notformed in the weld zone of steels of different grades due to the highcontent of Cr which is a ferrite former element.

In addition, comparative example No. 31 which had a small Si content of0.15% and a large Mo content of 0.4% was poor in terms of surfacequality and, in particular, in terms of corrosion resistance and surfacequality in the weld zone of steels of the same grade in the case of aninsufficient gas shield and in the weld zone of steels of differentgrades with SUS304 in the case of an insufficient gas shield.

Moreover, in the case of comparative example No. 32 which had a small Sicontent of 0.25% and a Nb content of 0.10% which was larger than therange according to preferred embodiments of the present invention,residual scale was found after rapid annealing had been performed in aline for carbon steel, which means that the base metal (after coldrolling, annealing and pickling had been performed) of No. 32 was poorin terms of corrosion resistance.

As described above, it has been clarified that, according to the presentinvention, a ferritic stainless steel with excellent corrosionresistance of base metal, corrosion resistance in the weld zone ofsteels of the same grade, corrosion resistance in the weld zone ofsteels of different grades with SUS304 and with excellent surfacequality of a cold-rolled, annealed and pickled sheet can be obtainedwithout grinding the surface of a hot-rolled, annealed and pickled steelsheet.

The present invention can be suitably used as a material for parts, ofwhich corrosion resistance is required, such as house wares, kitcheninstrument, architectural interior and exterior, metal fittings inarchitecture, the interior of an elevator and an escalator, electricappliances, and automobile components.

1. Ferritic stainless steel having a chemical composition containing, bymass %, C: 0.003% or more and 0.012% or less, Si: 0.30% or more and0.60% or less, Mn: 0.10% or more and 0.35% or less, P: 0.040% or less,S: 0.020% or less, Cr: 17.0% or more and 19.0% or less, Ni: more than0.10% and 0.30% or less, Ti: 0.10% or more and 0.40% or less, Nb: 0.005%or more and less than 0.050%, Mo: 0.20% or less, N: 0.005% or more and0.015% or less, Cu: 0.3% or more and 0.5% or less, Mg: less than0.0005%, and the balance being Fe and inevitable impurities.
 2. Theferritic stainless steel according to claim 1, the steel having thechemical composition further containing, by mass %, Al: 0.01% or moreand 0.5% or less.
 3. The ferritic stainless steel according to claim 1or 2, the steel having the chemical composition further containing, bymass %, Sb: 0.05% or more and 0.30% or less.
 4. The ferritic stainlesssteel according to claim 1, the steel having the chemical compositionfurther containing, by mass %, one or two of Zr: 0.01% or more and 0.60%or less and V: 0.01% or more and 0.50% or less.
 5. The Ferriticstainless steel according to claim 3, the steel having the chemicalcomposition further containing, by mass %, one or two of Zr: 0.01% ormore and 0.60% or less and V: 0.01% or more and 0.50% or less.
 6. Theferritic stainless steel according to claim 2, the steel having thechemical composition further containing, by mass %, Sb: 0.05% or moreand 0.30% or less.
 7. The ferritic stainless steel according to claim 2,the steel having the chemical composition further containing, by mass %,one or two of Zr: 0.01% or more and 0.60% or less and V: 0.01% or moreand 0.50% or less.